Photographic printer with automatic sensor calibration

Abstract

A photographic printer includes light sensors for measuring optical characteristics of photographic film to be printed. Prior to printing, the output signals of the light sensors are monitored under one or more reference condition, and calibration values based upon this monitoring are derived and stored. During normal operation of the photographic printer the sensors provide measurements of the optical characteristics of the film, and the output signals of the light sensors are calibrated based upon the stored calibration values. The exposures are then controlled as a function of the corrected output signals.

What is claimed is: 
     
       1. In a photographic printer system in which light is passed through a photographic film original to expose a photosensitive medium, the improvement comprising: a plurality of light sensors for providing output signals indicative of light received from defined areas of the photographic film original, the light sensors being positioned at a location along a path of the photographic film original prior to the location at which light is passed through the photographic film original to expose the photosensitive medium;   calibration value means for calculating digital calibration values for each of the plurality of light sensors based upon values of the output signals under a first reference condition in which light to the light sensors is blocked and a second reference condition in which light is received by the light sensors;   digital storing means for storing the plurality of digital calibration values;   correcting means for correcting the output signals from the plurality of light sensors during normal operation of the printer system based upon the stored plurality of digital calibration values; and   exposure control means for controlling exposures of the photosensitive medium as a function of the corrected output signals during normal operation of the printer system.   
     
     
       2. The invention of claim 1 wherein the digital calibration values include a digital offset calibration value for each output signal. 
     
     
       3. The invention of claim 1 wherein a filter of essentially uniform attenuation is placed in the path of light being received by the light sensing means during the second reference condition. 
     
     
       4. The invention of claim 3 wherein the filter of essentially uniform attenuation is unexposed, processed photographic film. 
     
     
       5. The invention of claim 1 wherein no filter is placed in the path of light being received by the light sensing means during the second reference condition. 
     
     
       6. The invention of claim 1 and further comprising: converter means for converting the output signals from analog to digital output signals.   
     
     
       7. The invention of claim 1 wherein the calibration value means, the correcting means, and the exposure control means comprise digital processor means. 
     
     
       8. The invention of claim 7 wherein the correcting means further comprises: gain-controlled amplifier means for amplifying the output signals; and   gain control interface means for controlling the gain of the gain-controlled amplifier means as a function of the digital calibration values.   
     
     
       9. The invention of claim 7 wherein the correcting means comprises: multiplying A/D converter means for converting the output signals from analog to digital values; and   A/d control interface means for controlling the converting by the multiplying A/D converter means as a function of the digital calibration values.   
     
     
       10. A method of controlling a photographic printer having a light sensor array for providing a plurality of output signals indicative of light sensed by the light sensor array from defined areas of a photographic film, the method comprising: blocking light to the light sensor array to produce a first reference condition;   permitting light to reach the light sensor array to produce a second reference condition;   deriving digital calibration values for each sensor of the array based upon the values of the output signals during the first and second reference conditions;   storing the digital calibration values;   correcting the plurality of output signals provided by the light sensing array during normal operation of the photographic printer based upon the stored digital calibration values; and   controlling exposures as a function of the corrected output signals.   
     
     
       11. The method of claim 10 wherein deriving calibration values comprises deriving an offset value for each sensor. 
     
     
       12. The method of claim 11 wherein permitting light to reach the light sensor array comprises interposing a filter of essentially uniform attenuation in the path of light to the light sensor array to produce the second reference condition. 
     
     
       13. The method of claim 12 wherein the filter of essentially uniform attenuation is unexposed, processed photographic film.